feat(render): R-A2 — per-building floods (the flap fix)

Replace the outdoor root's single unified reverse-portal flood (whose root-level
portal-side test oscillated as the chase eye grazed a doorway — the measured
flood 2<->6) with retail's per-building floods.

- OutdoorCellNode.Build(uint): portal-less land root; floods only itself ->
  full-screen OutsideView -> terrain (PortalVisibilityBuilder IsOutdoorNode seed).
- PortalVisibilityBuilder.ConstructViewBuilding: per-building flood seeded at a
  building's own finite entrance (retail ConstructView(CBldPortal) 0x5a59a0 via
  DrawPortal 0x5a5ab0 / portal_draw_portals_only 0x53d870). Entrance-bounded ->
  consistent ~2-cell depth (measured retail cell_draw_num, handoff OPTION-A 3.4).
- RetailPViewRenderer.DrawInside: when the root is the outdoor node, group nearby
  cells by BuildingId and merge each per-building flood into the frame before
  assembly; existing shells/object-list draw path unchanged. 48 m seed cutoff.
- GameWindow: pass flat NearbyBuildingCells only on outdoor-node frames.

Tests: +3 PortalVisibilityRobustnessTests (per-building touches ~2 cells, membership
stable under the measured 36 um eye jitter). UnifiedFloodTests retired (its subject,
the unified flood from the outdoor node, is removed); surviving full-screen-OutsideView
coverage moved to OutdoorCellNodeTests. App Rendering 207/207, Core movement 14/14.

Conformance-verified sound; the grazing-doorway flap is the visual acceptance test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/PortalVisibilityBuilder.cs

@@ -517,6 +517,25 @@ public static class PortalVisibilityBuilder
         return frame;
     }
 
+    /// <summary>
+    /// Retail per-building flood — <c>PView::ConstructView(CBldPortal*, …)</c> (decomp:433827),
+    /// reached from <c>BSPPORTAL::portal_draw_portals_only</c> (0x53d870) → <c>DrawPortal</c>
+    /// (0x5a5ab0) during the terrain BSP walk. Floods ONE building's cells from its outside-facing
+    /// entrance portal(s). Identical machinery to <see cref="BuildFromExterior"/>, but the CONTRACT is
+    /// per-building: the caller passes exactly one building's cells, so the seed is that building's
+    /// FINITE entrance opening (bounded flood depth → the stable ~2-cell view retail draws per visible
+    /// building, measured live §3.4). This differs from the synthetic outdoor node's single unified
+    /// flood whose full-screen-ish seed reaches variable depth into a building as the eye moves — the
+    /// 2↔6 oscillation. Robustness is validated by the conformance test, not assumed.
+    /// </summary>
+    public static PortalVisibilityFrame ConstructViewBuilding(
+        IEnumerable<LoadedCell> buildingCells,
+        Vector3 cameraPos,
+        Func<uint, LoadedCell?> lookup,
+        Matrix4x4 viewProj,
+        float maxSeedDistance = float.PositiveInfinity)
+        => BuildFromExterior(buildingCells, cameraPos, lookup, viewProj, maxSeedDistance);
+
     // The NDC [-1,1] viewport quad (CCW), reused by the flap probe's clip recompute.
     private static readonly Vector2[] FullScreenQuad =
         { new Vector2(-1f, -1f), new Vector2(1f, -1f), new Vector2(1f, 1f), new Vector2(-1f, 1f) };